The Classic Cadillac Forum Discussion, Late '70's Cadillac Low Beam/ High Beam Question... in Past Cadillac Vehicle Discussion; Hey everyone,
I just can't recall the 'normal' operation from so long ago, so I put this out there.
With ...

Re: Late '70's Cadillac Low Beam/ High Beam Question...

Thanks for checking on it, let me know what you find with your '79. I do have the the optional high output generator/alternator. It has correct and steady output, even under load.

Possibly a short in the switch... so i went on ebay and purchased a new floor mount dimmer switch and will install that when it arrives and see what happens afterwards. Certainly can't hurt to change it after 35yrs anyway.

Re: Late '70's Cadillac Low Beam/ High Beam Question...

The low beam bulb has 2 filaments. low beam and hi beam. The hi beams bulbs have one filament. Once the hi beams are selected the low beam bulbs should get brighter. Why don't you unplug the inboard hi beams and see if both filaments illuminate on the low beams when you engage hi beams. Could be 2 burnt out hi beam filaments in your outboard bulbs.

Re: Late '70's Cadillac Low Beam/ High Beam Question...

Originally Posted by carnut

The low beam bulb has 2 filaments. low beam and hi beam. The hi beams bulbs have one filament. Once the hi beams are selected the low beam bulbs should get brighter. Why don't you unplug the inboard hi beams and see if both filaments illuminate on the low beams when you engage hi beams. Could be 2 burnt out hi beam filaments in your outboard bulbs.

Hey carnut thanks for the info. I will check out this out and report back.

BTW... did you get to read the last detailed update on my old '1979 Seville help' thread? If not, you can check it out, but Bruce Roe tested & rebuilt my ECM with his custom designed MAP sensor, he also tweaked it using his bench test emulator. I then soldered in his under-dash external fuel pump relay bypass mod last weekend to protect the new ECU. Finally, to address the issue you were helping me with on the car, I found a like new fast idle switch on ebay and installed that as well. The car runs like new. There were big problems with the 35yo failing MAP sensor and my fast idle heater switch was not functioning properly. Even when it's warm out like now, the car still idles better & smoother on cold start than ever before, (A tad bit high right off, but kicks down quickly to a nice steady idle).

Just thought you'd like to know and thanks again for all the help on resolving issues. Mark~

but with projector lens inside and a removable superwhite H4 bulb (much brighter/whiter light), and this harness you found would make a nice new & clean install. I'm going to test the old original sealed beams per carnut, and then 2nd test with the old wires and new H4 bulbs... see if any diff, and go from there.

Will post findings and pictures of anything out of ordinary.

Thanks again everyone, I really appreciate everyone's help & input in this forum. Mark~

Re: Late '70's Cadillac Low Beam/ High Beam Question...

I tested the hi-beams on my '79 last night against the garage wall.
The low beams did not get any dimmer when the high beams came on, but they did change. I think, like carnut said, a different fillament lit up.

Re: Late '70's Cadillac Low Beam/ High Beam Question...

OK, so I went out to start testing some things and this time the dimmer switch is doing nothing at all, dead and the blue hi-beam crest indicator on the IP stays lit up all the time now. So... I pull back the carpeting and look what I found. I have done lots of work on this car myself, but never had reason to pull carpeting back before.

This certainly cannot be original factory wiring. The car only had 1 other PO who, for over 30yrs. only had the car serviced by his local Cadillac dealer from brand new.

This repair looks strange to me, I see no re-routing or mismatched wire colors (down here anyhow), why is it cut and re-connected right in line on all 3 wires? Did they remove something?

Maybe carnut knows, but was something like what you see here (quick-splice connectors) used regularly for electrical repairs by Cadillac Dealer Service Technicians?

I have never had any issues with electronic accessories, equipment or any shorting out problems. But, now that I stumbled upon this, I'm def gonna have to spend some time tracing out these wires and 4 headlamp wires to see if they all match up, (as you can see, they are all same color as dimmer switch wires), then discovering why this repair was done and working thru the replacement of the dimmer switch, etc 1 step at a time.

FYI, I do own the original 1979 Service Manual and the electrical/vacuum circuit diagrams to refer to.

Re: Late '70's Cadillac Low Beam/ High Beam Question...

I'm thinking that maybe the fuse box is melted at either end of the fuse and it was wired to jump across that melted area to make better contact to the fuse? I've seen many floor mounted dimmer switches with melted connections at the dimmer itself after years of use. Given the history you mentioned, I'm thinking the technician was trying to do a quicker repair to save time and it must have worked up till now. Look at the dimmer itself for a damaged connector. And yes, we are now "friends"

Re: Late '70's Cadillac Low Beam/ High Beam Question...

Hi Neil,

Well, the problem existed (and still does) with the original design sealed beams. But... thanks for posting the sheets, this is excellent info for me once everything is corrected and setup with all H4's.

For now, I have unplugged the brights and the H4 bulbs are working perfectly. I still intend on wire tracing and installing the new dimmer switch, then re-testing everything with all 4 bulbs active. From there, I'll see if I need that new harness. Mark~